Phase-locked loops are used in a multitude of communications and instrumentation applications performing phase-tracking operations such as phase-coherent demodulation, carrier tracking, timing recovery, bit synchronization, and Doppler measurement. Because of the trend toward digital signal processing, it is increasingly necessary to perform analog-to-digital conversion in addition to phase-tracking. Usually, this is done by combining traditional A/D converters with either analog or digital phase-locked loops. The drawback is that relatively precise analog circuitry is required for even moderate levels of digital conversion accuracy. The current invention employs a novel technique for simultaneously performing phase-tracking and A/D conversion with the goals of reducing analog circuit complexity and sensitivity to non-ideal component behavior. Theoretical and computer simulation results indicate that systems based on the technique can be developed that have comparable performance to the traditional approach with significantly less complex and precise analog circuitry.
The technique gives rise to a family of systems that will be referred to as delta-sigma frequency-to-digital converters (.DELTA..SIGMA.FDCs). Like a phase-locked loop, each .DELTA..SIGMA.FDC tracks the phase and frequency of its input signal. However, unlike a phase-locked loop, it performs coarse analog phase measurements using a one-bit A/D converter (i.e., a hard limiter) sampled at many times the Nyquist rate of the signal bandwidth, and employs quantization noise shaping and decimation filtering to obtain an accurate digital estimate of the instantaneous frequency of its input. Accordingly, .DELTA..SIGMA.FDCs operate on instantaneous frequency in a similar manner to the way delta-sigma (.DELTA..SIGMA.) modulator-based A/D converters operate on amplitude. Hence, they share many of the benefits enjoyed by .DELTA..SIGMA. modulator-based A/D converters such as reduced analog circuit requirements and amenability to very large scale integration (VLSI) implementation.